1. Field of Invention
The invention relates to preform elements faced with superhard material, and particularly to preform cutting elements, for use in drag-type rotary drill bits, comprising a facing layer of superhard material bonded to a substrate of material which is less hard than the superhard material.
2. Description of Related Art
Preform elements of this kind are often used as cutting elements on rotary drag-type drill bits, and the present invention will be particularly described in relation to such use. However, the invention is not restricted to cutting elements for this particular use, and may relate to preform cutting elements for other purposes. For example, elements faced with superhard material of the kind referred to may also be employed in workpiece-shaping tools.
Preform elements used as cutting elements in rotary drill bits are generally in the form of circular or part-circular tablets comprising a flat facing table of superhard material, the rear surface of which is bonded, in a high pressure high temperature press, to the front surface of a less hard substrate. The front facing table is usually of polycrystalline diamond, although other superhard materials are available, such as cubic boron nitride and amorphous diamond-like carbon (ADLC). The substrate is often formed from cemented tungsten carbide.
In order to mount the cutting element on the body of a drill bit, the rear surface of the substrate of the element is often bonded, for example by brazing, to a carrier in the form of a stud or post, often of cemented tungsten carbide. Part of the carrier is then received in a socket in the bit body, and secured by brazing or shrink-fitting, so as to hold the cutting element at the appropriate orientation to the bit body and hence to the surface of the formation being drilled. Usually the cutting element is orientated so that part of the outer periphery of the facing table forms a cutting edge which engages the formation, so that during use a wear flat develops extending at an angle across the facing table and substrate, and perhaps also part of the carrier.
In an alternative arrangement the substrate is of sufficient axial length that it may, itself, be secured within a socket in the bit body.
Other forms of cutting element are known where the superhard facing layer is applied around the peripheral surface of a generally cylindrical, as shown for example in U.S. Pat. No. 5,279,375.
In this type of cutting element a cylindrical part of the element, which may or may not include part of the peripheral superhard facing layer, is secured within a socket in the bit body. During use of the cutting element the cutting face of the element comprises an area of substrate, exposed at one end of the element and surrounded by an annular surface of superhard material, comprising the end edge of the peripheral superhard layer. Arrangements are also known where the exposed part of the cutting element is frusto-conical so as to reduce the area of the element which forms the cutting face.
The present invention relates to certain improvements in preform cutting elements of the last-mentioned kind and to methods of mounting such elements, as well as other types of preform elements on the body of the drill bit.